1. Field of the Invention
The present invention relates to an apparatus for supplying breathable gas to a human, used in, for example, Continuous Positive Airway Pressure (CPAP) treatment of Obstructive Sleep Apnea (OSA), other respiratory diseases and disorders such as emphysema, or the application of assisted ventilation.
2. Description of Related Art
CPAP treatment of OSA, a form of Noninvasive Positive Pressure Ventilation (NIPPV), involves the delivery of a pressurized breathable gas, usually air, to a patient's airways using a conduit and mask. Gas pressures employed for CPAP can range, e.g., from 4 cm H2O to 30 cm H2O (typically in the range of 8-15 cmH2O), at flow rates of up to 180 L/min (measured at the mask), depending on patient requirements. The pressurized gas acts as a pneumatic splint for the patient's airway, preventing airway collapse, especially during the inspiratory phase of respiration.
Typically, the pressure at which a patient is ventilated during CPAP is varied according to the phase of the patient's breathing cycle. For example, the ventilation apparatus may be pre-set, e.g., using control algorithms, to deliver two pressures, an inspiratory positive airway pressure (IPAP (e.g., 4-8 cm H2O)) during the inspiration phase of the respiratory cycle, and an expiratory positive airway pressure (EPAP (e.g., 10-20 cm H2O)) during the expiration phase of the respiratory cycle. An ideal system for CPAP is able to switch between IPAP and EPAP pressures quickly, efficiently, and quietly, while providing maximum pressure support to the patient during the early part of the inspiratory phase.
In a traditional CPAP system, the air supply to the patient is pressurized by a blower having a single impeller, i.e., a single stage blower. The impeller is enclosed in a volute, or housing, in which the entering gas is trapped while pressurized by the spinning impeller. The pressurized gas gradually leaves the volute and travels to the patient's mask, e.g., via an air delivery path typically including an air delivery tube.
Other blowers utilize a pair of impellers with, for example, one on either side of the motor but fixed to a common output shaft. Such configurations are disclosed in commonly-owned U.S. Pat. No. 6,910,483 and in commonly-owned co-pending application Ser. No. 10/864,869, filed Jun. 10, 2004, each incorporated herein by reference in its entirety.
Single-stage blowers are often noisy and are not as responsive as two-stage blowers in that they require longer periods of time to achieve the desired pressure. Two-stage blowers tend to generate less noise since they can run at lower speeds to generate the desired pressure, and are more responsive. On the other hand, two stage or double-ended blowers tend to be too large for certain applications.